Pneumatic hybrids

Running on air

A cheaper way to increase fuel efficiency in cars

BEING green can be expensive, as any driver of a Toyota Prius can tell you. The car is a hybrid, combining an electric motor that powers it at low speeds in the city with a petrol motor that is useful for accelerating and cruising at high speeds. It is the most efficient car sold in America, but it costs upwards of $22,000, a price that can wipe out the savings on fuel. One reason for the price is that the car contains expensive batteries. Another is that the transmission system had to be completely redesigned. But there may be a cheaper and altogether simpler way to make a hybrid that avoids this—using air power instead of electricity.

Using compressed air to power a car has one obvious disadvantage: compressed air has a low energy density, so the stored energy gets used up quickly. However, in urban driving, this may be more than compensated for using regenerative braking. One of the most important roles of the batteries in a hybrid is to store energy recovered when the car brakes. The idea with a pneumatic hybrid is to store this energy as compressed air. Such a vehicle would run on petrol but would use its reservoir of compressed air to boost the power when needed. This would not demand a serious redesign, because every car already has a makeshift air compressor in the form of the engine itself. Building a pneumatic-hybrid car would thus be relatively cheap.

When the driver of a pneumatic-hybrid car applied the brakes, the fuel supply to the engine would be shut off while the pistons that normally propel the vehicle would help to slow it, pumping pressurised air into a tank as they did so. According to Lino Guzzella of the Swiss Federal Institute of Technology in Zurich, who has built a prototype pneumatic-hybrid engine, this sort of set up could reduce fuel consumption by 32% compared with a normal engine. It would thus offer 80% of the fuel savings of an electric-hybrid vehicle at a lower price.

Using this set up would also allow smaller, more efficient engines to offer the “driving experience” of the large engines currently used. These engines are large so that they can accelerate a vehicle rapidly: their power is not needed for pootling along. It would be more efficient to have small engines that used turbochargers to provide the extra power for acceleration. Today's turbochargers, however, take several seconds to deliver the extra oomph. In a pneumatic hybrid engine, compressed air could act as an immediate turbocharger, forcing more air into the combustion chamber during acceleration. This would allow more fuel to be burned (since there is more oxygen available to burn it with) and would thus generate power for acceleration.

Pneumatic-hybrid cars are best suited to city driving, according to John Heywood of the Massachusetts Institute of Technology. He says the compressed-air tank needs to be kept topped up by regular braking. On motorways, drivers brake less often.

Stop-start city driving is particularly suited to one application of these hybrids. Hua Zhao of Brunel University, in London, has been developing a variation of the pneumatic-hybrid engine for lorries and buses. Some commercial vehicles are already designed to use their engines as compressors that slow them down and prevent the brakes from overheating when they are descending long hills. Other vehicles use air-driven starter motors. Dr Zhao reckons he can make a pneumatic-hybrid engine using these existing components. But instead of injecting compressed air into the combustion chamber, Dr Zhao's design would use the compressed-air tank to replace the electrically powered compressors that feed air starter motors. Simulations of this design suggest it would significantly reduce fuel consumption, says Dr Zhao, who is now looking for a commercial partner to build a test vehicle.